The passive optical network (PON) system is an optical fiber access technology. It is a high-speed broadband access technology that can offer high-speed communication at low costs. Normally, the passive optical network (PON) system, which includes an optical line terminal (OLT), an optical network unit (ONU), and an optical distribution network (ODN), provides end users with a broadband access method, and has many advantages over other present broadband access technologies. The most conspicuous advantage is that the passive optical network, for example, a GE-PON system can provide end users with access speeds of gigabit level, and the end users can accommodate more satisfactorily to broadband networks.
A single-core optical fiber is used for a line between OLT and ONU, and a downstream signal (communication direction from OLT to each ONU) and an upstream signal (communication direction from ONU to OLT) are sent and received with wavelength 1490 nm and 1310 nm, respectively.
Downstream signals to individual ONUs from OLT are transmitted specifying a destination ONU by the optical broadcaster system, and an ONU selectively receives a signal addressed to it. Conversely, upstream signals to OLT from individual ONUs, to avoid collision of the signals, are transmitted by ONU permitted for transmission by OLT after adjusting transmission timing by the time division multiple access system (TDMA system).
The dynamic bandwidth allocation (DBA) system, which is known as a method for adjusting transmission timing, determines an upstream allocated bandwidth of each ONU and transmission timing from each ONU, based on transmission requests from all ONU.
By the way, in OLT, when receiving an optical signal transmitted from ONU, an avalanche photo diode (APD) having optical multiplication effect is used. The APD can receive signals of weak optical power with high sensitivity by multiplication effect by a reverse-biased PN junction part. Generally, by setting a current multiplication factor of APD large for weak signals, and setting it small for intense signals, reverse-bias voltage of APD are adjusted. In other words, to increase a current multiplication factor (M value) of APD, a reverse-bias voltage is set large, and to reduce a current multiplication factor of APD, a reverse-bias voltage is set small. Usually, a reverse-bias voltage is adjusted and set when a device is introduced. In a PON system, however, since optical power signals from individual ONUs are different, the dynamic range of optical receiver sensitivity of an optoelectronic transducer of OLT must be widened. Although APD can receive signals of weak optical power with high sensitivity because of multiplication effect by the reverse-biased PN junction part, once a multiplication factor is set by applying a reverse-bias voltage, the range of optical receiver sensitivity of APD is determined. It is known that the range of optical receiver sensitivity becomes narrow when a high-speed optical signal is received, and in a high-speed PON system, there is a problem that a signal of intense optical power and a signal of weak optical power cannot be received at the same time. A detailed description is made below.
FIG. 8 is a schematic diagram of a PON system in a related art. OLT 10 and plural ONUs 20 are connected by an optical fiber via light splitters 30a and 30b. Usually, OLT is installed in a station office of a carrier, and ONUs are installed in home.
The operation of the PON system is briefly described. A downstream packet inputted to the OLT 10 from an upper network is converted into a PON frame in the OLT 10, and an optical signal is broadcast to the ONUs 20 via the optical fiber 30. The optical signal is branched in the light splitters 30a and 30b before being transmitted to the ONUs 20. An ONU having a matching transmission destination address captures the optical signal, and processes the packet for decoding.
On the other hand, an upstream signal transmitted from the ONUs 20 is transmitted to the OLT 10 via the light splitter 30a. In the OLT 10, an upstream signal received in the APD 15 is converted into a current signal, converted into a voltage signal via a transimpedance amplifier 16, and inputted to a post amplifier 17. For the inputted signal, a PON frame is decoded in PHY 18, and subjected to predetermined processing in a MAC frame processing unit 19 before being outputted to the upper network. The OLT 10 is provided with a DC current source 14 of voltage variable type that applies a reverse-bias voltage to the APD. A satisfactory receiver sensitivity is set by applying a reverse-bias voltage usually at the time of introduction.
Upstream packets transmitted from the ONUs 20 must be avoided conflicting with each other in terms of time. Therefore, CPU 11 provided in the OLT 10 determines upstream transmission timing by the dynamic bandwidth allocation system (DBA). An ONU 20 determined for transmission timing sends an upstream packet at the allocated time. As a result, collision of upstream packets among the ONUs is avoided. Although the OLT and the ONUs must share time, this can be done by including time information in downstream packets.
When the OLT receives optical signals from the ONUs, since the number of splitters existing between the OLT and the ONUs, and the distance of the optical fiber differ for different ONUs, the received signals are different in optical power.
FIG. 10A is a drawing showing differences of optical power among upstream packets from the ONUs, and takes optical power in a vertical axis and time in a horizontal axis. A packet from the ONU 20a, a packet 803 from the ONU 20b, and a packet 802 from the ONU 20c are received, and are different in reception optical power. The cycle of a signal of each packet is about one nanosecond.
FIG. 9 shows an example of a level diagram and optical receiver sensitivity in the PON system. In the PON system, to accommodate differences of transmission distance and the number of branches, a wide dynamic range of optical receiver sensitivity is required. For example, the IEEE802.3 standards dictate about 20 dB. However, when a high-speed optical signal is received, for example, during high-speed transmission such as 10 Gbps, the light-receiving range of APD when a multiplication factor is set by applying a reverse-bias voltage is smaller than 20 dB. As shown in FIG. 9, when a reverse-bias voltage Va of the APD is set for a signal of intense optical power, there has been a problem that optical signals of weak optical power cannot be received.
FIGS. 10B and 10C show reverse-bias voltage and output currents of the APD, and take time in a horizontal axis. As shown in FIG. 10B, in the PON system, a reverse-bias voltage 811 is set constant in terms of time. At this time, as shown in FIG. 10C, output currents 821 and 822 of the APD satisfy a current range in which the transimpedance amplifier 16 and following circuits operate normally. However, an output current 823 of the APD does not fit in the current range in which they operate normally. Specifically, the OLT 10 can receive signals 821 and 822 of intense optical power, but cannot receive a signal 823 of weak optical power. As described above, although the development of high-speed technology for PON is rapid, since the dynamic range of the optical receiver sensitivity of very high-speed photoelectric conversion elements is small, signals different in optical power cannot be received at the same time, so that requirements for the PON system cannot be satisfied. As a countermeasure for such a problem, when an optical signal transmitted from an ONU is received, a reverse-bias voltage corresponding to each ONU is set in the APD in line with reception timing of a signal permitted for the ONU. This technology is disclosed in, for example, JP-A-2005-45560.
To facilitate the setting of threshold for determining the level of light-receiving signals, the order of adjacent time slots is controlled so that a change in light-receiving intensities in the time slots becomes small, based on the light-receiving intensities of optical signals. This technology is disclosed in, for example, JP-A-2004-15243.